I agree with KN4MKB. I’ve been hosting my own mail server for decades. Not one issue. I use that in lieu of a mail service provider (Google immediately comes to mind), as their EULA service agreement will tell you that - since you’re using their service, on their servers - anything goes. Read the fine print on Gmail, and you’ll see. 😉
Also, check out “ciphermail”. It’s end-to-end encryption mail server.